A copolymer obtained by adding a polybutadiene to styrene monomer to conduct radical polymerization is widely known as an impact-resistant polystyrene resin composition which combines the excellent properties inherent in polystyrene and improved impact resistance.
Modifier rubbers generally used for producing the impact-resistant polystyrene resin composition include a low-cis polybutadiene obtained by polymerizing 1,3-butadiene using an alkyllithium catalyst and made up of from 30 to 35% cis-1,4 structures, from 10 to 20% vinyl structures, and from 50 to 60% trans-1,4 structures (hereinafter referred to as "low-cis BR") and a high-cis polybutadiene obtained by polymerizing 1,3-butadiene using a cobalt, titanium, or nickel catalyst and made up of from 90 to 98% cis-1,4 structures, from 1 to 5% vinyl structures, and from 1 to 5% trans-1,4 structures (hereinafter referred to as "high-cis BR").
On the other hand, it has been reported that a high-cis high-vinyl BR (hereinafter referred to as "HC-HV BR") was obtained with a catalyst comprising a combination of methyl aluminoxane (hereinafter referred to as "MAO") and a titanium metallocene complex, butoxy titanate compound, or transition metal acetylacetone complex, i.e., an MAO-CpTiCl.sub.3 [CpTi(OBu).sub.3, Ti(OBu).sub.4 ] catalyst system [see Macromol. Symp., 89, 383(1995)], with an MAO-CpVC1.sub.2 (Cp.sub.2 VCl) catalyst system [see Polymer, 37, 363(1996)], or with an MAO-Cr(acac).sub.3 [V(acac).sub.3, Fe(acac).sub.3 ] catalyst system [see IRC-95 Kobe, Preprint, 25C-4(1995)]. However, all these prior art catalysts have low activity, and a low polymer concentration results.
In JP-B-62-16201 is reported an impact-resistant polystyrene resin containing an HC-HV BR obtained using a cobalt catalyst in combination with a dihydric alcohol. (The term "JP-B" as used herein means an "examined Japanese patent publication".) However, this technique is unsuitable for industrial use because the HC-HV BR has low catalytic activity and the polymer concentration is low.
Furthermore, it has been reported in JP-B-56-50894 and JP-A-55-129403 that an HC-HV BR was obtained even in the presence of an organophosphorus compound. (The term "JP-A"as used herein means an "unexamined published Japanese patent application".) However, these techniques each is unsuitable for practical use because the catalytic activity is low and the polymer concentration is also low.
In JP-A-6-80855, it has been reported that a thermoplastic resin composition excellent in impact resistance and tensile elongation was obtained from a styrene/butadiene copolymer rubber, a rubber-modified styrene/methyl methacrylate graft copolymer, and a styrene/butadiene block copolymer. However, this prior art composition is not always satisfactory.
A feature of high-cis BR's resides in that although excellent in low-temperature properties because of the low glass transition temperature thereof (usually from -95 to -110.degree. C.), they have low reactivity with styrene monomer (low grafting percentage) because of the low vinyl structure content thereof. Therefore, the impact-resistant polystyrene resin compositions obtained using high-cis BR's are unsatisfactory in the size reduction of rubber particles (gloss) and surface impact resistance (Du Pont impact resistance), although excellent in Izod impact resistance.
On the other hand, low-cis BR's have a high glass transition temperature (usually from -75 to -95.degree. C.) and have high reactivity with styrene monomer (high grafting percentage) because of the high vinyl structure content thereof. Consequently, impact-resistant polystyrene resin compositions obtained using low-cis BR's are unsatisfactory in Izod impact strength and low-temperature properties, although excellent in the size reduction of rubber particles and in surface impact resistance.
There has recently been a strong desire for the development of a BR which can retain both the desirable properties of high-cis BR's and those of low-cis BR's when the BR is used as a modifier rubber of an impact-resistant polystyrene resin composition and which can improve properties of the impact-resistant polystyrene resin composition such as impact resistance, low-temperature properties, gloss, tensile strength, and tensile elongation in the well balance.